A conventional fan with a sound muffling box, which is a fan encased in a sound muffling box such as a ventilation fan, reduces suction noise as follows. A flow-dividing sound muffler is provided on the rear face of the fan casing of a double suction fan encased in a sound muffling box. Suctioned air flow is divided into two streams: one flowing through a motor-side air passage and the other through an opposite-to-motor-side air passage. The amount of air drawn into each air passage is adjusted to make the sound muffling material have an effective sound-absorbing function (see, for example, Patent Literature 1).
The reduction of the suction noise of the fan with the sound muffling box will now be described with reference to FIG. 10, which is a plan sectional view of the conventional fan with the sound muffling box. As shown in FIG. 10, in the conventional fan, box 101 includes fan casing 103 with impeller 102, and double suction centrifugal fan 105 with motor 104.
When impeller 102 rotates, air is drawn through box inlet port 106 of box 101, passes through double suction centrifugal fan 105, and exhausted from box outlet port 107. On the rear face of fan casing 103, there is attached flow-dividing sound muffler 108. Flow-dividing sound muffler 108 divides the air drawn through box inlet port 106 into two streams: one flowing through motor-side air passage 109 and the other through opposite-to-motor-side air passage 110.
Flow-dividing sound muffler 108 has an opening portion 111 formed near opposite-to-motor-side air passage 110 on the box inlet port 106 side. The opening portion 111 enables a larger amount of air to flow into opposite-to-motor-side air passage 110 than into motor-side air passage 109. Motor-side air passage 109 is prone to cause flow noise because air flow is obstructed by motor 104 and motor mounting leg 112. The generation of this flow noise, however, is reduced by forcing a larger amount of air flow to flow through opposite-to-motor-side air passage 110.
Thus, in this conventional fan with the sound muffling box, the flow noise caused in motor-side air passage 109 is suppressed by dividing the suctioned air flow into plural streams at the rear face of fan casing 103. However, in order to achieve the intended objective of reducing the suction noise in box 101, it is necessary to make flow-dividing sound muffler 108 large enough as a sound-muffling air passage. This requires enlarging the distance between box inlet port 106 and the rear face of fan casing 103, thereby increasing the material cost. Furthermore, it becomes difficult to miniaturize box 101.
In a fan with a sound muffling box designed as a type to handle a large amount of air flow, box inlet port 106 is generally connected to a rectangular duct. In this case, the cross-sectional area of the air passage of the rectangular duct, i.e. the opening area of box inlet port 106 is larger than the projected area of the rear face of fan casing 103 viewed from box inlet port 106. Therefore, part of the air flow drawn through box inlet port 106 reaches double suction centrifugal fan 105 without colliding with the rear face of fan casing 103, causing flow-dividing sound muffler 108 to be ineffective.